The role of inhomogeneities in nanowire superconducting single-photon detectors

Abstract

Nanowire superconducting single-photon detectors (SSPDs) are promising devices for single-photon detection applications at telecom wavelengths, in terms of high efficiency, low dark count rate and short dead time. They consist of a long, 100 nm wide, superconducting niobium nitride nanowire folded into a meander pattern and biased close to the critical current. Photons incident on the SSPD locally break the superconductivity which results in a resistive barrier across the nanowire. The voltage drop across this barrier is measured. The devices presented in this thesis are fabricated on a gallium arsenide substrate: a material system that allows integration of single-photon sources and detectors on a single chip. Such quantum photonic integrated circuits (QPICs) are of major importance for scalable quantum computing.Measurements on nominally identical SSPDs show large variations in critical current and efficiency of the devices . This is a huge problem for applications where arrays of SSPDs are needed such as linear optical quantum computing. The authors of [1] attribute the observations to geometrical constrictions: regions in the nanowire where the cross section is reduced. This reduces the critical current of the wire and the SSPD cannot be biased efficiently anymore. In this thesis, it is shown that the continuously distributed inhomogeneity of the nanowire dimension or the crystal structure, instead of localized constrictions, is the origin of the limited detection efficiency and poor uniformity of our SSPDs.To investigate the effect of inhomogeneities, SSPDs with lengths in the range from 100 nm to 15 µm are fabricated and characterized electrically and optically. The two main observations from the electrical characterization are a decrease of the critical current with increasing length and a decrease of the spread in critical current values with increasing length. These measurements also indicate that the length scale of the inhomogeneities is 100 nm or smaller. The main result of the optical characterization is a decrease of the detection efficiency with increasing wire length.Simple assumptions on how the inhomogeneities influence the critical current of the wires allowed us to create a numerical model that quantitatively reproduces the measured variations of the critical current. This result is used to model also the detection efficiency of the SSPDs. This model qualitatively reproduces the general trends in the efficiency measurements, although some deviations are observed for the longer wires.

Date of Award	30 Apr 2014
Original language	English
Supervisor	Andrea Fiore (Supervisor 1)